The primary safety consideration in the operation of any nuclear reactor is the control and containment of radioactive material and radioactivity, under both normal and accident conditions. Numerous controls and barriers are installed in nuclear plants to protect workers and the public from the effects of radiation.
Regular inspection of nuclear reactors is essential to avoid catastrophic breakdowns and to properly schedule preventive maintenance procedures. In boiling water reactors (BWRs), one area that requires inspection is the steam dryer assembly. The steam dryer assembly is mounted in the reactor vessel above the steam separator assembly to form the top and sides of the wet steam plenum. Wet steam flows upward and outward through the dryers. Moisture is removed by impinging the wet steam on the dryer vanes, and the condensed moisture flows down through drains to the reactor water. Although the steam dryer is not a safety-related component, the assembly is designed to withstand design basis events without the generation of loose parts, and the dryer is expected to maintain structural integrity.
A major challenge to the inspection of steam dryers is that they are in the shape of an inverted can with the “top” being closed. Access for inspection is gained through relatively narrow openings near the bottom of the structure, so the inspection device must travel upward after passing through the openings.
Most attempts to address this problem have involved the use small Remote Operated Vehicles (ROVs) or other tethered vehicles. The PHANTOM® Firefly™ product from Deep Ocean Engineering is one such commercially-available ROV camera system designed for inspection of BWR reactors. Like most ROVs, this system uses a combination of buoyancy and thrusters to maneuver the vehicle within the vessel. This can be problematic because of the many obstacles and small spaces created by the internal hardware within the structure, which the vehicle must navigate around with an umbilical trailing behind it. Moreover, difficulty has been experienced in keeping a freely movable ROV in a given position for a period long enough to inspect a given region with a camera. Improvements thus would be desirable.